grab video using a video adapter rather than getting rgb pixels from pixmap

Video players will try accelerated adapters first (xvideo, etc) then fallback to painting RGB. Since we want to compress the video as a video stream, there is no point having the video players convert their stream to RGB for us to convert it back to YUV straight after for encoding!

One solution to this is a Wayland-Xpra (#387), but Wayland isn't ready for prime time yet and won't cover X11 applications well enough (I don't think), at least for now.

The flash player doesn't use libva apparently, so #510 is not an option there, but maybe we can use a technique similar to ​libvdpau-va-gl and capture the YUV stream even before it is scaled up.

I'm not sure how userspace would then register/grab the YUV data from the dummy-Xv adapter, but this would make #410 (better sub-window encoding: detect regions and use sub video encoder) much easier to implement.